1. Field of the Invention
The present invention relates to a novel fluorinated cation exchange membrane. More particularly, it relates to a fluorinated cation exchange membrane having an ion exchangeable group which is effectively usable for diaphragm electrolysis of an aqueous solution of electrolytes such as an alkali metal chloride.
2. Description of the Prior Art
It has been known to produce alkali metal hydroxide in a cathode compartment and chlorine in an anode compartment by partitioning the anode compartment and the cathode compartment and electrolyzing an aqueous solution of alkali metal chloride fed therein. The method has been known as a diaphragm electrolysis in two compartments. Heretofore, an asbestos diaphragm has been usually used in the method. When the asbestos diaphragm is used, the asbestos itself is corroded by the electrolyte. Accordingly, the asbestos diaphragm disadvantageously has a short lifetime.
In this method, the diaphragm has fine pores which pass the electrolyte whereby alkali metal chloride is incorporated in the resulting alkali metal hydroxide to cause low purity. Furthermore, when the concentration of alkali metal hydroxide is increased, the current efficiency is disadvantageously decreased.
In order to prolong the life of the diagragm and to use it over a long time period, it has been proposed to coat the asbestos diaphragm with an alkali resistant fluorinated resin having a hydrophilic group or to form a diaphragm made of the alkali resistant fluorinated resin having the hydrophilic group (U.S. Pat. Nos. 3,853,720; 3,853,721). The diaphragm used in the latter method is also porous thereby passing the electrolyte. Accordingly, the purity of the alkali metal hydroxide obtained is relatively low. When the concentration of the alkali metal hydroxide is increased, it is difficult to prevent the current efficiency from decreasing. It has also been proposed to use a cation exchange membrane which does not substantially pass the electrolyte and selectively passes alkali metal ions as the diaphragm. It has been especially proposed to use a fluorinated cation exchange membrane having high alkali resistance and chlorine resistance (U.S. Pat. Nos. 3,773,634; 3,852,135). When such a cation exchange membrane is used as the diaphragm, the electrolyte is not substantially passed through it but only alkali metal ions are selectively passed through it, whereby the contamination of the alkali metal chloride in the resulting alkali metal hydroxide can be prevented to obtain a product having a satisfactory purity. However, there does not exist a cation exchange membrane having satisfactory current efficiency. Even the best known cation exchange membranes such as one made of a copolymer of C.sub.2 F.sub.4 and CF.sub.2 = CFOCF.sub.2 CF(CF.sub.3)OCF.sub.2 CF.sub.2 SO.sub.2 F can not attain a current efficiency higher than 85% in a concentration of sodium hydroxide of higher than 20%, though the current efficiency may be higher when the concentration of sodium hydroxide is low.